In our latest YouTube episode we had a potential collision with a ship which we monitored using AIS. The ship was on the same course as us, coming up fast on our stern and seemingly oblivious to our position. One of the questions frequently discussed in the boating world is which is better, AIS or radar? In this post we will explain how they work, and answer that question. Controversial? You decide…
The last fifteen years has seen a huge growth in popularity of the AIS system both in the commercial and leisure sectors. With AIS a relatively cheaper option compared to radar, people new to boating often make the mistake of thinking it is an either/or option. Meanwhile many old-school sailors don’t see the need for AIS when radar offers a truer picture of what is in front of the boat. The fact is both are incredibly useful tools.
What is AIS?
- AIS stands for Automatic Identification System.
- It traditionally uses a VHF transceiver and a GPS receiver, and is an automatic tracking system for avoiding collisions at sea.
- There is also the satellite-based S-AIS, system, but for now we’ll focus on what the average cruiser uses, the cheaper Class B system, which first became available in 2006.
- AIS uses VHF at 2W, is output on marine band channels 87 and 88, and the range is restricted to around 5-10 miles.
- It’s used primarily to monitor local traffic, but the emergence of internet-based services offers world-wide viewing of AIS targets.
Advantages of AIS?
- The beauty of AIS is the amount of information that each target broadcasts:
- name of the vessel, its MMSI number, position, course, and speed,
- quick calculations like the Closest Point of Approach (CPA).
- All this information can be overlaid onto a chart plotter or via a dedicated screen.
- As well as avoiding collisions, the technology can also used for:
- fishing fleet monitoring,
- cargo tracking,
- search and rescue,
- aids to navigation
- and even accident investigation.
- Targets can frequently be seen behind solid objects like mountains or harbour walls.
FOLLOW US WITH AIS
App: On Course by Marine Traffic
SY Esper MMSI number: 235026188
Disdvantages of AIS?
- Not everyone has it
- You may be transmitting on your AIS, but unless the other vessel has AIS they won’t necessarily see you
- The IMO (International Maritime Organisation) stipulates that AIS is fitted to all international voyaging ships with 300 or more gross tonnage, and all passenger ships regardless of size. So that means a lot of boats are not required to be installed with AIS: Leisure boats, fishing fleets, dinghies…
- Not all passenger ships are fitted with AIS. And then, of course, each country will have different rules.
- Not everyone turns on their AIS – You are depending on the skipper of the other vessel to turn on their AIS. Maybe they forgot, maybe their unit is faulty. There could be security reasons why they don’t turn it on.
What happened on that near collision?
- In this week’s episode we were monitoring the ship on AIS. We could see that it was almost directly behind us, travelling at 12kn.
- THE CPA calculation was around 100m or less. The target on screen, indicated by a triangle with the longest point indicating the direction, also displays a forward-pointing dotted line. This can be configured to show the distance it will travel after any number of minutes you choose.
- In our case our AIS targets are set to show where the ship will be at the end of the dotted line after ten minutes. Because AIS broadcasts a ship’s MMSI number and name, we were able to hail the boat and discuss collision avoidance.
- In this case the skipper wasn’t so co-operative, but on the whole verbal communication between two vessels potentially on a collision is recommended, and is normally positive.
What is RADAR?
- Radar stands for ‘radio detection and ranging’, and has been around a lot longer than AIS.
- It uses radio waves to determine the angle, bearing, range and velocity of an object.
- It was developed in the Second World War, although it was German physicist Heinrich Hertz who discovered radio waves could bounce of an object back in 1886.
Advantages of RADAR
With all the information available from AIS, why bother with radar?
- RADAR doesn’t lie. It works on a simple principle: what you see is what you get.
- If there is an object in front of you, whether or not it is lit at night or in poor visibility, the radar should pick it up. (Of course there are exceptions like adverse weather conditions, but if you see an object on radar that isn’t an echo, chances are it will be a solid object.)
- Radar will show you where the land is.
- Radar will show you where it’s raining and help you track the speed and direction of a squall.
- Sometimes, if conditions are good, it will pick up other objects, like fishing pots/flags and floating debris.
- It is a BIG aid during night watches.
Disadvantages of RADAR
- It won’t give you the name of the ship,
- Your boat’s movement affects the targets, and they move around on the screen, so calculations like CPA, which you can do with radar, are sometimes spurious because of the movement of the boat.
- It’s range is only line-of-sight.
- It can suffer from interference, both internal and external.
- Sometimes there is too much “clutter” – unwanted echoes bouncing off the target
- It can be jammed, intentionally or by accident (if another person is operating equipment using the same frequency range).
- Bigger the vessel, the larger the target – conversely, smaller vessels can be more difficult to see, if at all.
- Radar can only show you the solid object, not the vessel behind it.
RADAR for yachties
- Until recently radar required its own screen and did not use data protocols like NMEA to share information to other systems like chart-plotters (but this has now changed).
- Traditionally radar was power-intensive but again, with the development of 4G radar, we’ve got around this problem.
- Radar is usually a lot more expensive than AIS.
Which is better?
Neither! Both radar and AIS can be invaluable in accessing a potential collision threat and we absolutely recommend using the two together.
- We often overlay both radar and AIS on our chart plotter and it’s always interesting to see what each picks up. It’s easy to fall into the trap of thinking the boat ahead is the AIS target you see on your plotter when the radar could be telling another story.
- There is no such thing as ‘right of way’. Stand on/stand off, but ultimately exercise caution at the earliest opportunity and avoid ships as best you can, irrespective of who is supposed to be the stand on vessel.
- We should emphasize that collision avoidance should start with your eyes. Do not rely solely on electronic equipment.
- Don’t be afraid to get on the radio to ensure the vessel you can see has seen you. So call up that 300m cargo ship on ch16 when it’s ten miles away. AIS gives you the name and the MMSI number so if you don’t get a response on 16 you can always use digital selective calling if you have it. We find most commercial skippers to be accommodating and professional.
If we had to choose ONLY ONE, which would it be?
Do you agree? We would love to hear your thoughts in the comments below.
Peace and fair winds!
Liz, Jamie and Millie xxx
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